95 Suburban throwing DTC78

[dieselrob]

Have been getting the flashing on and off SES the last couple of trips. I have noticed it seems down on power and have seen a little black smoke on acceleration and up hills. Have not checked the vacuum system yet as I just got home a little bit ago and it is midnight. Code says vacuum solenoid but I read somewhere that 78 means overboost only. Dont see how that could be with less power and black smoke. Tell me what the deal is.

 

[JMJNet]

78 is Turbo Wastegate solenoid fault. It is very common.

1. Check for leak vacuum line.
2. If there is no leak, change the Turbo Solenoid located on the driver side.
3. There is a small chance that vac pump is bad, in which case you may want to get a TurboMaster.

 

[Missy Good Wench]

The OBD1 system is pretty stupid and can only check the electrical portion of the system.

With a 78 showing its one of two things. The connector/connection is fouled or the solenoid (little critter on the back of the LH VC) is bad.

Unless the plug or wiring has been molested its likely the solenoid.

Always check the vacuum to the solenoid 26"hg and steady
Check the vacuum to the waste gate actuator 20" hg and steady at idle

If the source vacuum from the vacuum pump is not steady (Needle very wiggly) toss the pump and get a fresh one.

OR ash can the whole thing and use a Turbo master and a Heath chip.

I have the TM and chip OH My how lovely that is.

Good luck

MGW

 

[Turbine Doc]

You are experincing the OBD-I version of this http://dieseltowingresource.com/showthread.php?t=10974 same troubleshooting along with my final recommendation to scrap the vac system, another achillies heel IMO of the 6.5, is reliabilty of the vac system once things start getting old, never seem to fail until you are hooked up to a big load, or you need to get down the highway, many folks don't realize the error of driving the heck out of a truck with bad WG, to make up for the power loss, black smoke = heat and lots of it, with a WG fail dead on road best bet is to drive conservatively with the WG wired fully shut.

Best option however is run a non wg turbo, something like the ATT, now might just be the time to justify it, hey truck is 14 years old, vac system is starting to fail, ATT has better mpg anyway will pay for itself, better power,

decisions/decisions buy the ATT or fix the vac system, use the GM set up use more fuel and matter of when not if it will crap out again on a 14+ yr system next time it fails, if I still had a GM turbo, I'd wire wg shut for now and start saving for the ATT

 

[JiFaire]

if I still had a GM turbo, I'd wire wg shut for now and start saving for the ATT

You're braver than me... I have always been a little leery of wiring the darn thing shut...

However, you can easily build yourself a TM for under $5 - it won't be the precision instrument that Heath sells, and you will definitely need a boost gauge with it, but you can go that way while you save up for ATT. The other thought I've seen is a 'helper spring' on the wastegate arm... to help close the WG if the vacuum tails off. Looks pretty mickey-mouse, but it works.

 

[buddy]

You could also try putting vaccum pump line straight to the wastegate acuator until you get a new solenoid.

If your sub is an F engine, which I think most or all were, then there is no vac sensor, but an S engine is a little smarter and actually does have a vac sensor for the EGR and could tell you low vacuum. Not that you would ever want an S engine with EGR.

So since you dont have EGR vacuum sensor, it must be an electrical problem or solenoid is not responding to PCMs requests.

 

[dieselrob]

It was the solenoid. I could pop it with a screwdriver and get vacuum at the turbo but it would not cycle vacuum but only a few seconds. Tried the WD40 trick to no avail. Bought a new one yesterday and it works again.

How much low end throttle response do I lose with the ATT? I like the sound of everything about it but the turbo lag. Is there another turbo out there that has better low end and maybe not as much on the top?

 

[JiFaire]

It was the solenoid. I could pop it with a screwdriver and get vacuum at the turbo but it would not cycle vacuum but only a few seconds. Tried the WD40 trick to no avail. Bought a new one yesterday and it works again.

Good diagnosis and fix, Rob!

My Da used to tell me that if you couldn't fix it, get a bigger hammer It's amazing how many things smarten up when you tap 'em with a tool... sort of like saying "That's for nothing... guess what happens NEXT, if you don't smarten up?"

 

[Turbine Doc]

You're braver than me... I have always been a little leery of wiring the darn thing shut...

.

Well I can let it out of the bag now that I'm not doing it no more, I ran fully closed with vac direct plumbed, (same as wired shut), then when vac pump died I installed a TM fully hogged down (same as wired shut), for about 3 years between the 2 methods.

But I was IC'd and running with gauges, which is why I said wire closed until you could fix it but drive conservatively., this by any means is not a substitute method for the long term situation does need to be corrected, by some normalized regulation method, or replace with a non wg turbo.

That said, you have to be running hard or towing big with wired shut, full vac, or hogged down TM to make more than 10 psi boost, if driven conservatively, you still have protection from the PCM with defuel, especially if not fooling the MAP(boost) sensor. This to me is far better than limping around a truck/burb with no boost, but it does not eliminate need to remove this band aid temporary fix

 

[Turbine Doc]

It was the solenoid. I could pop it with a screwdriver and get vacuum at the turbo but it would not cycle vacuum but only a few seconds. Tried the WD40 trick to no avail. Bought a new one yesterday and it works again.

How much low end throttle response do I lose with the ATT? I like the sound of everything about it but the turbo lag. Is there another turbo out there that has better low end and maybe not as much on the top?

Now that I have fixed my slippage in my trans, I'm confident that was some of the contribution to felt lag, now I'd say the lag is nil & you would not miss a thing, if there is a better turbo I've not seen any substantive/documented reporting on it.

Holsets do good job, but make so much more boost PSI wise, that to be safe you need to lower the CR of the engine. Beauty of the ATT for me anyway is that it's operating envelope is same as what GM turbos are, so it's a bolt on instal, better everything, there are about 16 or 17 of them out running now, no complaints that I've seen with lag other than my initial results, which after the trans overhaul I just did, I'm backing away from.

There is some lag (very slight) as you do get a bIgger plume of black at first spool up (clears fast, & truck is crisply accelerating) when I was with the gm turbo, but that only occurs if I stomp it; with normal acceleration I have no discernable change in the exhaust plume, if you lok at data in other thread you almost have a 1:1 ratio boost to drive pressure, which is what you really want to see in a turbo. My Burb is next one to get one, probalby gonna happen next week, before I go on vacation to Va. if I get one missing fitting I need.

Before I hijack it any more we probably should be discussing this in the replacement turbo thread.

Glad to see it was the WG solenoid that fixed it for you. "We love it when a plan comes together" Jim where is that pic of Hannibal Smith when you need it?

 

[Matt Bachand]

For those worrying about lag, don't forget boost doesn't start to build until 1500 or so anyway, so i wouldn't worry about it. By the time you need the boost, its there. This is based on GM-8 experience with a boost guage.

I may be off with RPM range, so don't call me on it, but the point is, it doesn't start to build boost instantly like a supercharger may.

What is desireable is good boost with low backpressure to make the boost. That is where the ATT shines. Unless you're drag racing from the line, you want the powerband available when you're already cruising, so Unless you downshift and WOT, I wouldn't worry twice about lag.